I am trying this tool out and following the tutorials and got a different result than suggested by the tutorial.I was doing the Slab section and when I selected Symmetry the slab disappeared.I have attached the before and after screen shots.Am I doing something wrong?

The Curve must not cross itself when you make a Slab. In this case the starting curve slightly crosses the centre line, when it is made symmetrical it is as if a copied curve is placed on the other side, and they overlap a bit - breaking the Slab.

To fix, use the Move/Soft Move tool on the curve to drag it slightly away from the centre line and the Slab should appear.